Elastomers, also known as rubber materials, have found application in a diverse range of areas including tyres, shoe soles, condoms, surgical and examination gloves, catheter balloons, mining equipment and shock adsorption for bridges just to name a few. In the majority of cases, successful adoption of elastomers in these and other applications has required some modification of the base elastomer to improve their mechanical properties, including the use of particulate or fibrous materials introduced into the elastomer matrix to act as reinforcing agents. Silica in particulate form is in widespread use to reinforce elastomers, typically providing increased strength, stiffness and toughness to a base elastomer. Newer materials being investigated as reinforcing agents for elastomers include carbon nanotubes, graphene and nanoclay particles.
In the last few decades, the use of natural fibres to reinforce polymer composites has been increasing because of their sustainability, renewability, biodegradability, low thermal expansion, manufacturer-friendly attributes such as low density and abrasiveness, excellent mechanical properties such as very high specific stiffness and strength and consumer-friendly attributes such as lower price and higher performance. A typical natural fibre consists of several or more nanocrystalline elementary fibrils formed by cellulose chains (homopolymers of glucose), concreted by/in a matrix containing lignin, hemicellulose and other components. The nanofibrils consist of monocrystalline cellulose domains linked by amorphous domains. Amorphous regions act as structural defects and can be removed under acid hydrolysis, leaving cellulose rod-like nanocrystals, which are also called whiskers, and have a morphology and crystallinity similar to the original cellulose fibres. Depending on the source of cellulose, the cellulose content varies from 35 to 100%. These fibers exhibit extraordinarily higher mechanical properties (stiffness/strength) at nanoscale phases than at the microscale or in their natural state. In recent years, these nanocrystalline cellulose fibres have been explored as biologically renewable nanomaterials that can be applied in several engineering applications.
While numerous methods have been explored for the production of microfibrillated cellulose (MFC), which by definition (Reference: Robert J. Moon, Ashlie Martini, John Nairn, John Simonsen and Jeff Youngblood, ‘Cellulose nanomaterials review: structure, properties and nanocomposites’ Chem. Soc. Rev., 2011, 40, 3941-3994), consists of cellulose fibres with diameters in the range of 20-100 nm and a length range of between 0.5 and tens of μm, the production of nanofibrillated cellulose (NFC; also known as cellulose nanofibres CNFs)), and cellulose nanocrystals (CNCs), is more challenging due to the requirement to separate or deconstruct the cellulose fibres to a much greater degree. Attempts to date to produce these two types of nanocellulose (CNCs and NFCs), have focussed on the use of chemical, physical, mechanical and enzymatic pretreatments alone or in combinations thereof. For NFC, the prior art refers to a fibre diameter in the range of 3-20 nm and a length in the range between 0.5 and 2 μm. For CNC, the prior art refers to fibre diameters in the range of 3-20 nm and length up to 500 nm (except the special example of tunicate CNCs or t-CNCs, which have a higher aspect ratio).
Although both CNCs and NFCs are nanocellulose materials, they exhibit different morphologies. CNCs are typically highly crystalline rod shaped particles with typical dimensions ranging from 5 to 20 nm in diameter and from 100 to 500 nm in length. On the other hand, NFCs have a diameter typically in the range of 5 to 20 nm and lengths within the micron scale. NFCs might be considered to be bundles of elementary cellulose fibrils (also known as primary cellulose fibrils or primary cellulose nanofibrils) embedded in a (primarily) hemicellulose matrix.
Nanocellulose materials have been the subject of a number of research studies in which the nanocellulose was used in the production of a composite material. For example, Abraham et al, Physicomechanical properties of nanocomposites based on cellulose nanofibre and natural rubber latex, Cellulose, (2013), 20:417-427, published 22 Nov. 2012, describes the use of NFCs having a diameter of 10 to 60 nm and obtained by the steam explosion of banana fibre as a reinforcing material mixed with natural rubber latex. This paper described loadings of NFCs added to the latex as including 2.5%, 5%, 7.5% and 10%. Table 2 of this paper shows that adding the NFCs to the natural rubber latex resulted in significant increases in elastic modulus (stiffness), and tensile strength and a decrease in elongation (strain) at break across all loadings of NFC.
Boufi et al, “Mechanical performance and transparency of nanocellulose reinforced polymer nanocomposites, Macromol. Mater. Eng. 2014, 299, pp 560-568, describes the use of two different types of NFC (one having 20 to 50 nm diameter, 200 to 1000 nm length, the other having 10 to 20 nm diameter, 200 to 1000 nm length) and two different types of CNC (one having 15 to 25 nm diameter, 150 to 250 nm length and the other having 15 to 25 nm diameter, 150-350 nm length) being dispersed in an acrylic latex at up to 15% loading. The nanocellulose materials were extracted from two different cellulosic sources, namely alfa and date palm trees. This paper found that a huge enhancement in modulus (stiffness) was observed in the polymer composite above the glass transition temperature (rubber state). Further, this paper states that the stiffness of the composites increases with an increase in the aspect ratio of the CNC.
Chaker et al, “Reinforcing potential of nanocellulose from non-woody plants,” Polymer Composites—2013, describes the use of a number of NFCs derived from different sources (abaca, sisal, hemp, jute and flax) as fillers in an acrylic elastomer (a commercially available latex obtained by the copolymerisation of styrene (35% by weight) and butyl acrylate (65% by weight)). The NFCs had diameters in the range of 10 to 50 nm. The NFCs were described as comprising a bundle of primary nanofibrils of 3 to 5 nm diameter. The NFCs had a hemicellulose content that ranged from 6 to 20% weight. NFC loadings in the elastomer/NFC composite were up to 15% by weight. This paper observed a huge enhancement in modulus above the glass transition temperature of the elastomer. The paper states that this is a common effect seen in elastomer composites reinforced with nanocellulose. This paper also described composites including fibres with higher hemicellulose content as showing higher stiffness and tensile strengths compared to lower hemicellulose materials. FIG. 5 of this paper showed significantly increased stiffness and reduction in elongation to break on increasing NFC loadings.
US 2012/0232192 A1 discloses rubber composites incorporating organic fibres for improving the performance of automotive tyres. This document teaches the use of modified celluloses such as carboxymethyl cellulose (CMC) as an additive rather than unmodified celluloses, explaining that the presence of hydroxyl groups on unmodified cellulose promotes aggregation of the cellulose in water due to strong hydrogen bonding, resulting in poor dispersion in the rubber. The document goes on to describe modified cellulose particles that have an average diameter in the range 20 microns to 100 microns, teaching that particles with a diameter lower than 20 microns have poor dispersibility.
US 2013/0197132 A1 describes the incorporation of microfibrils of cellulose into elastomers including rubber latex. Cellulose microfibrils with diameters as low as 20 nm are disclosed in the examples however the document teaches against the use of unmodified cellulose as a reinforcing agent in rubber composites due to the poor compatibility of unmodified cellulose fibres with the rubber component. Specifically, poor adhesion at the rubber-cellulose interface is thought to contribute to increased friction and energy losses at the interface. The authors propose the addition of lignin to the cellulose fibres to modify the rubber-cellulose interface to increase adhesion at the interface.
Although increases in tensile strength and toughness are useful in a very large number of applications, in some uses, concomitant increases in elastic modulus or stiffness may not be so desirable. Reinforcing agents that have been previously used in elastomers to improve mechanical properties such as silica, carbon black, carbon nanotubes and graphene can provide an increase in tensile strength of an elastomer however due to their rigid nature, the incorporation of these materials also tends to increase the hardness and stiffness of a material and reduces the elongation to break. In many elastomer applications, increased stiffness and reduced elongation to break are detrimental to product performance such that the use of these reinforcers involves a fine balancing act and compromise between different performance features. For example, in the manufacture of condoms, it is desirable to use an elastomeric material that has good tensile strength and toughness but low elastic modulus (or high compliance). This combination of properties will result in a condom that is resistant to breakage but also allows for a close fit and improved feel or sensation. The use of elastomeric materials having undesirably highly elastic modulus is generally avoided in condom manufacture because those materials tend to decrease feel or sensitivity, and therefore cause consumer resistance.
Previous research studies focussed on using reinforcing agents to improve the mechanical properties of elastomers manufactured from latex-based systems such as natural rubber lattices and polyisoprene lattices have largely used the casting method to fabricate the elastomer composite products. Here, the reinforcing agent is added to the latex, followed by casting the mixture onto a surface and subsequently the carrier solvent is removed, leaving a composite of the elastomer and the reinforcing agent. However, commercial products such as condoms, gloves and catheter balloons that are made using latex-based elastomers are often not made by casting, but are instead made by dipping a shaped form or mould into latex followed by removal of the mould from the latex and then drying of the latex film deposited on the mould to yield a latex film or membrane of the desired shape. In such cases, the reinforcing agent that is present in the latex must not only provide improvements in the mechanical properties of the final product following dipping, but in order that the latex-reinforcing agent formulation is suitable for manufacture, the addition of the reinforcing agent must not destabilise the latex colloid so as to cause precipitation of the latex. Thus, in order to provide a manufacturable latex formulation that results in a suitably reinforced dipped product, the reinforcing agent must be well dispersed into the latex but not cause significant destabilisation. It is generally known that the extent of colloid destabilisation is approximately proportional to the amount of reinforcing agent added to the latex. Therefore, only very effective reinforcing agents will be successful in this application, that is, those reinforcing agents that can provide adequate mechanical reinforcement in the manufactured device but in quantities small enough so as not to destabilise the latex.
In other elastomer applications such as wear liners, seals and tyres similar technical requirements exist. In wear liners for example, elastomeric materials that exhibit high toughness and abrasion resistance are desired while still retaining the soft, elastic nature of the underlying rubber material. In tyres, softer compounds provide good road grip however these tend to wear more rapidly that harder compounds. Increasing the hardness of a tyre rubber compound reduces wear but at the expense of road grip. There is needed therefore, a soft rubber compound that provides good road grip while possessing good resistance to wear.
It will be clearly understood that, if a prior art publication is referred to herein, this reference does not constitute an admission that the publication forms part of the common general knowledge in the art in Australia or in any other country.